1. Field of the Invention
The present invention relates to a melt supply pipe for supplying a molten aluminum alloy from a melting furnace to a plunger sleeve of a die casting machine in aluminum die casting.
2. Background Art
In conventional die casting machines, a ladle method has been commonly employed for supplying a molten aluminum alloy to a plunger sleeve. According to the ladle method, a molten aluminum alloy is drawn from a melting furnace by means of a ladle and supplied to a plunger sleeve.
As a technique to take the place of the ladle method, a melt supply pipe method has recently been attracting attention which involves directly connecting a melting furnace and a plunger sleeve with a melt supply pipe, and supplying a molten aluminum alloy through the melt supply pipe to the plunger sleeve. Mixing of an Al oxide film or solid broken pieces into a molten aluminum alloy can be significantly reduced with the melt supply pipe method as compared to the conventional ladle method. The melt supply pipe method thus has the advantage that it can provide a higher-quality die-cast product.
A conventional melt supply pipe, which has so far been used to connect a melting furnace and a plunger sleeve, has such a structure that a heater is wrapped around a ceramic pipe. A ceramic material is used for a melt supply pipe because the material has high melting loss resistance to a molten aluminum alloy.
While a ceramic pipe is thus strong to a molten aluminum alloy, it is weak to impact and can be broken by its vibration during operation or by erroneous handling upon its maintenance. Further, only an insufficient load can be applied on the connecting portions of such a breakable ceramic pipe, which could cause leakage of a molten aluminum alloy from the connecting portions.
The applicant has proposed a molten aluminum alloy-contact member having enhanced melting loss resistance to a molten aluminum alloy, comprising a steel base, a Ni alloy layer formed on the steel base, and TiC bonded in a particulate state to the surface of the Ni alloy layer (Japanese Patent Laid-Open Publication No. 2005-264306).
Further, a melt supply pipe is known which has such a structure that a ceramic or graphitic pipe is encased in a steel pipe for the purpose of covering the breakableness of the inner pipe. However, because of a large difference in thermal expansion coefficient between the steel pipe and the ceramic or graphitic pipe, a large gap can be formed between the inner and outer pipes due to their different thermal expansions. A molten aluminum alloy will easily intrude into the gap, which may result in melting loss of the steel pipe and formation of holes therein in a short period of time.